This document summarizes monitoring Kubernetes clusters with the prometheus-operator. It introduces the prometheus-operator project, describes the main components it provides like Prometheus, Alertmanager, ServiceMonitor and PodMonitor custom resources. It explains how these resources work and how the operator configures and deploys monitoring targets. It also introduces the kube-prometheus project which provides manifests to easily monitor a Kubernetes cluster out of the box. Finally it provides tips on troubleshooting and where to find help and documentation for using the prometheus-operator.